Method and apparatus for operating buffer state report in wireless communication system

ABSTRACT

A method of supporting a buffer status report (BSR) associated with a device-to-device (D2D) communication includes: establishing a radio resource control (RRC) connection with an evolved NodeB (eNB) and receiving configuration information associated with a D2D communication from the eNB, the configuration information including information of a resource allocation mode for a D2D data transmission and information of a timer associated with a BSR for a D2D data transmission; determining, by a user equipment (UE), a radio link failure (RLF), the RLF being associated with a connection problem of the RRC connection; initializing a re-establishment process for the RRC connection; canceling all BSRs for a D2D data transmission in response to the determined RLF, the canceled BSRs for a D2D data transmission being associated with a D2D data transmission scheduled by the eNB; and initializing the timer associated with a BSR for a D2D data transmission.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/279,002, filed on Feb. 19, 2019, which is a continuation of U.S.application Ser. No. 14/820,443, filed on Aug. 6, 2015, which claimspriority from and the benefit of Korean Patent Application Nos.10-2014-0102128, filed on Aug. 8, 2014, and 10-2014-0153775, filed onNov. 6, 2014, all of which are hereby incorporated by reference in theirentirety.

BACKGROUND 1. Field

The present disclosure relates to wireless communication, and moreparticularly, to a method and an apparatus for operating a buffer statereport when a user equipment performs a device-to-device communicationusing a resource allocated by a base station in a wireless communicationsystem supporting device-to-device communication.

2. Discussion of the Background

Device-to-device (D2D) communication relates to a communication methodhaving been available since the era of an analog two-way radio and hasbeen used over the very long history. However, D2D communication in awireless communication system is distinguished from existing D2Dcommunication.

The D2D communication in the wireless communication system indicatescommunication that uses a transmission and reception technology, forexample, a physical channel, of the wireless communication system in afrequency band of the wireless communication system or a band excludingthe frequency band, and in this instance, enables user data to bedirectly transmitted and received between devices, for example, userequipments (UEs) without using an infrastructure, for example, anevolved-NodeB (eNB). That is, two UEs function as a source and adestination of data, respectively, and perform communication. Such amethod enables wireless communication to be available in an area outsidea limited wireless communication infrastructure and also decreasesnetwork load of the wireless communication system. For D2D communicationin a wireless communication system, a base station may scheduleresources needed for in-coverage UEs to transmit data via D2Dcommunication. To this end, a UE may notify the base station through abuffer state report (BSR) of how much data is in a UE buffer to betransmitted via D2D communication.

In an exceptional case, a UE may transmit D2D data through a resourcethat the UE autonomously selects, instead of being allocated by the basestation a resource necessary to transmit data via D2D communication. Forthis case, when an already triggered BSR is present in the UE, it needsto be determined how the UE handles the already triggered BSR before,and there is also a need for a detailed BSR operation and definition ofthe BSR.

SUMMARY

Exemplary embodiments relate to a method and apparatus for operating abuffer state report for supporting a Device-to-Device (D2D)communication.

Exemplary embodiments also relate to a method and apparatus fordetermining a resource allocation for a Device-to-Device (D2D)communication.

Exemplary embodiments also relate to a method and apparatus for stoppinga buffer state report of a Device-to-Device (D2D) communication withcanceling BSR for a D2D data transmission based on a determined RLFcondition for system efficiency.

An exemplary embodiment provides a method of supporting a buffer statusreport (BSR) associated with a device-to-device (D2D) communication, themethod including: establishing a radio resource control (RRC) connectionwith an evolved NodeB (eNB) and receiving configuration informationassociated with a D2D communication from the eNB, the configurationinformation including information of a resource allocation mode for aD2D data transmission and information of a timer associated with a BSRfor a D2D data transmission; determining, by a user equipment (UE), aradio link failure (RLF), the RLF being associated with a connectionproblem of the RRC connection; initializing a re-establishment processfor the RRC connection; canceling all BSRs for a D2D data transmissionin response to the determined RLF, the canceled BSRs for a D2D datatransmission being associated with a D2D data transmission scheduled bythe eNB; and initializing the timer associated with a BSR for a D2D datatransmission.

An exemplary embodiment provides a user equipment to support a bufferstatus report (BSR) associated with a device-to-device (D2D)communication, the UE including: a wireless transceiver to establish aradio resource control (RRC) connection with an evolved NodeB (eNB) andto receive configuration information associated with a D2D communicationfrom the eNB, the configuration information including information of aresource allocation mode for a D2D data transmission and information ofa timer associated with a BSR for a D2D data transmission; and aprocessor configured to determine a radio link failure (RLF), the RLFbeing associated with a connection problem of the RRC connection, toinitialize a re-establishment process for the RRC connection, to cancelall BSRs for a D2D data transmission in response to the determined RLF,the canceled BSRs for a D2D data transmission being associated with aD2D data transmission scheduled by the eNB, and to initialize the timerassociated with a BSR for a D2D data transmission.

An exemplary embodiment provides a method of supporting a buffer statusreport (BSR) associated with a device-to-device (D2D) communication, themethod including: establishing a radio resource control (RRC) connectionwith an evolved NodeB (eNB) and receiving configuration informationassociated with a D2D communication from the eNB, the configurationinformation including information of a resource allocation mode for aD2D data transmission; operating a user equipment (UE) in a firsttransmission mode for a D2D communication, the first transmission modecorresponding to the resource allocation mode; determining, by the UE,an exceptional case; transitioning from the first transmission mode to asecond transmission mode in response to determining the exceptionalcase, the second transmission mode corresponding to a resource selectionmode in which the UE selects a resource for transmitting D2D data from aresource pool; and canceling at least one triggered BSR for a D2D datatransmission in response to determining the exceptional case, thecanceled BSR for a D2D data transmission being associated with a D2Ddata transmission scheduled by the eNB.

According to an exemplary embodiment, in D2D communication in a wirelesscommunication system, a UE which allocates a resource for D2Dcommunication or a UE present in a service coverage of a base stationmay efficiently be allocated a resource for D2D communication.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a wireless communication systemaccording to an exemplary embodiment.

FIG. 2 schematically illustrates a concept of D2D communication based ona cellular network according to an exemplary embodiment.

FIG. 3 illustrates a process of transmitting ProSe-BSR according to anexemplary embodiment.

FIGS. 4 to 8 are flowcharts illustrating a method for operating a bufferstate report for D2D communication by a UE according to an exemplaryembodiment.

FIG. 9 is a flowchart illustrating a method for operating a buffer statereport for D2D communication by a base station according to an exemplaryembodiment.

FIG. 10 is a block diagram illustrating a wireless communication systemaccording to an exemplary embodiment.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Exemplary embodiments will be described more fully hereinafter withreference to the accompanying drawings, in which exemplary embodimentsof inventive concept are shown.

Throughout the drawings and the detailed description, unless otherwisedescribed, the same drawing reference numerals are understood to referto the same elements, features, and structures. In describing theexemplary embodiments, detailed description on known configurations orfunctions may be omitted for clarity and conciseness.

Further, the terms, such as first, second, A, B, (a), (b), and the likemay be used herein to describe elements in the description herein. Theterms are used to distinguish one element from another element. Thus,the terms do not limit the element, an arrangement order, a sequence orthe like. It will be understood that when an element is referred to asbeing “on”, “connected to” or “coupled to” another element, it can bedirectly on, connected or coupled to the other element or interveningelements may be present. The present specification provides descriptionsin association with a wireless communication network, and tasks executedin the wireless communication network may be performed in the processwhere a system (for example, a base station) that manages thecorresponding wireless communication network controls the network andtransmits data, or may be performed in a User Equipment (UE) that iswireless linked to the corresponding network and capable ofcommunicating with the network system.

The present specification provides descriptions in association with acommunication network, and tasks executed in the communication networkmay be performed in the process where a system (for example, a basestation) that manages the corresponding communication network controlsthe network and transmits data, or may be performed in a User Equipment(UE) that is linked to the corresponding network.

FIG. 1 is a diagram illustrating a network architecture of a wirelesscommunication system, according to one or more exemplary embodiments.

Referring to FIG. 1, a wireless communication system 10 may provide acommunication service between a Base Station (BS) and a User Equipment(UE). In a wireless communication system, a UE and a BS may wirelesslytransmit and receive data. Also, the wireless communication system maysupport Device-to-Device (D2D) communication between UEs. The wirelesscommunication system that supports the D2D communication will bedescribed later.

A BS 11 of the wireless communication system 10 may provide acommunication service to a UE existing in a transmission coverage of theBS 11, through a predetermined frequency band. The coverage within whicha BS provides a service is also referred to as a site. The site mayinclude various areas 15 a, 15 b, and 15 c, which may be referred to assectors. The sectors included in the site may be identified based ondifferent identifier from one another. Each sector 15 a, 15 b, and 15 cmay be construed as a part of the area that the BS 11 covers.

A base station 11 communicates with User Equipment (UE) 12 and may bereferred to as eNB (evolved-NodeB), BTS (Base Transceiver System),Access Point, femto base station, Home nodeB, relay and Remote RadioHead (RRH). User equipment 12 (mobile station, MS) may be located at acertain location or mobile, and may also be referred to as differentterms, including UE (user equipment), MT (mobile terminal), UT (userterminal), SS (subscriber station), wireless device, PDA (personaldigital assistant), wireless modem, and handheld device. A cellinclusively refers to various coverage areas, such as mega cell, macrocell, micro cell, pico cell, and femto cell. A cell may be used as aterm for indicating a frequency band that a BS provides, a coverage of aBS, or a BS.

Hereinafter, the term downlink refers to communication from a basestation 11 to a UE 12, and the term uplink refers to communication froma UE 12 to a base station 11. For downlink, a transmitter may be part ofa base station 11, and a receiver may be part of a UE 12. For uplink, atransmitter may be part of a UE 12 and a receiver may be part of a basestation 11. There is no limitation in the multiple access method appliedto a wireless communication system. Diverse methods can be used,including CDMA (Code Division Multiple Access), TDMA (Time DivisionMultiple Access), FDMA (Frequency Division Multiple Access), OFDMA(Orthogonal Frequency Division Multiple Access), SC-FDMA (SingleCarrier-FDMA), OFDM-FDMA, OFDM-TDMA, OFDM-CDMA. Uplink transmission anddownlink transmission can use either TDD (Time Division Duplex), whichuses different time locations for transmissions, or FDD (FrequencyDivision Duplex), which uses different frequencies for transmissions.

The layers of a radio interface protocol between a UE and a BS may beclassified as a first layer (L1), a second layer (L2), and a third layer(L3), based on three low layers of an Open System interconnection (OSI)model in association with a communication system. A physical layerbelonging to the L1 among the layers, provides a information transferservice using a physical channel.

The physical layer is connected to a Media Access Control layer which isa higher layer, through a transport cannel. Data is transferred througha transport channel between the MAC layer and the physical layer. Thetransport channel is classified based on a scheme of transmitting datathrough a radio interface. In addition, data is transferred through aphysical channel between different physical layers (that is, betweenphysical layers of a UE and an eNB).

The physical channel may be modulated based on an Orthogonal FrequencyDivision Multiplexing (OFDM) scheme, and uses a space formed of time andfrequencies, and a space formed of a plurality of antennas as radioresources.

For example, a Physical Downlink Control CHannel (PDCCH) among physicalchannels may inform a UE of resource allocation of a Paging CHannel(PCH) and a DownLink Shared CHannel (DL-SCH) and Hybrid Automatic RepeatRequest (HARQ) information associated with a DL-SCH, and may deliver, toa UE, uplink scheduling grant which reports resource allocation ofuplink transmission. A Physical Control Format Indicator CHannel(PCFICH) informs a UE of the number of OFDM symbols used for PDCCHs, andis transmitted for each subframe. A Physical Hybrid ARQ IndicatorCHannel (PHICH) carries a HARQ ACK/NACK signal as a response to uplinktransmission. In addition, a Physical Uplink Control CHannel (PUCCH)delivers HARQ ACK/NACK with respect to downlink transmission and uplinkcontrol information such as a scheduling request and a Channel QualityIndicator (CQI). A Physical Uplink Shared CHannel (PUSCH) delivers anUpLink Shared CHannel (UL-SCH). The PUSCH may include HARQ ACK/NACK andChannel State Information (CSI) such as a CQI.

A Data Link Layer, which is the second layer of the OSI model, includesa Radio Link Control (RLC) layer and a Packet Data Convergence Protocol(PDCP) layer.

The MAC layer may execute mapping between a logical channel and atransport channel, and execute multiplexing or demultiplexing between atransport channel of a MAC Service Data Unit (SDU) that belongs to thelogical channel and a transport block provided in a physical channel.The MAC layer provides services to a Radio Link Control (RLC) layerthrough the logical channel. The logical channel is classified into acontrol channel for transferring control area information and a trafficchannel for transferring user area information. For example, servicesprovided from the MAC layer to a higher layer include data transmissionor radio resource allocation.

The functions of the RLC layer include concatenation, segmentation, andreassembly of an RLC SDU. The RLC layer provides three types ofoperation modes, such as, a Transparent Mode (TM), an UnacknowledgedMode (UM) and an Acknowledged Mode (AM), to secure various Quality ofServices (QoS) required by a Radio Bearer (RB).

The function of a Packet Data Convergence Protocol (PDCP) layer in theuser plane includes user data transmission, header compression, andciphering, and control plane data transmission and ciphering/integrityprotection.

An RRC layer controls a logical channel, a transport channel, and aphysical channel, in association with configuration, reconfiguration,and release of RBs. An RB indicates a logical path provided by a firstlayer (PHY layer) and a second layer (MAC layer, RLC layer, and PDCPlayer), for transferring data between a UE and a network. A process ofconfiguring an RB indicates a process that defines properties of radioprotocol layer and a channel for providing a predetermined service, andsets corresponding detailed parameters and an operation method. An RBmay be classified into a Signaling RB (SRB) and a Data RB (DRB). The SRBis used as a path for transmitting an RRC message and a Non-AccessStratum (NAS) message in the control plane, and the DRB is used as apath for transmitting user data in the user plane. Hereinafter, theterms “SRB” and “DRB” are collectively referred to as the term “RB,”which is to mean a DRB.

FIG. 2 illustrates the concept of cellular network-based D2Dcommunication applied according to an exemplary embodiment.

D2D communication refers to a technology which enables directtransmission and reception of data between UEs. Hereinafter, it isassumed that a UE supports D2D communication in an exemplary embodiment.The term “D2D” may be replaced with the expression “proximity-basedservice (ProSe)” or “ProSe-D2D.” The term “ProSe” is used for D2D tomean that a proximity-based service may be added to the technology whichenables direct transmission and reception of data between UEs, withoutchanging the meaning of the technology.

Recently, methods for carrying out discovery and direct communicationbetween in-coverage or out-of-coverage devices in a network have beenstudied for the purpose of public safety, etc. A UE performing D2Dcommunication may be referred as a D2D UE. Further, a UE transmitting asignal based on a D2D communication may be referred to as a transmitting(Tx) UE, and a UE receiving a signal based on a D2D communication may bereferred as a receiving (Rx) UE. A Tx UE may transmit a discoverysignal, and an Rx UE may receive the discovery signal. A Tx UE and an RxUE may exchange their roles therebetween. Further, a signal transmittedby a Tx UE can be received by two or more Rx UEs.

The load at a base station can be distributed and reduced if adjacentUEs perform D2D communications in a cellular system. Also, when adjacentUEs carry out D2D communications, UE's transmission power consumptionand transmission latency may be reduced because UEs send data to atarget located within a relatively short distance. Moreover, from theperspective the whole system, frequency utilization effectiveness isenhanced because existing cellular-based communication and D2Dcommunication use the same resources.

D2D communication may be classified into a communication method ofin-coverage UE, which is located in network coverage (base stationcoverage) and a communication method of out-of-coverage UE, which islocated out of network coverage.

Referring to FIG. 2, the communication between a first UE 210 located ina first cell and a second UE 220 located in a second cell and thecommunication between a third UE 230 located in a first cell and afourth UE 240 located in a first cluster may be D2D communicationbetween UEs in a network coverage. The communication between the fourthUE 240 located in the first cluster and a fifth UE 250 located in thefirst cluster may be D2D communication between the UEs located outside anetwork coverage. The fifth UE 350 is the cluster header, and thecluster header may operate as an independent Synchronization Source(ISS) for synchronization of an out-of coverage UE.

The D2D communication may include a discovery process that executesdiscovery for communication between UEs and a direct communicationprocess in which UEs transmit and receive control data and/or trafficdata. The D2D communication may be used for various purposes. Forexample, D2D communication within a network coverage and D2Dcommunication outside a network coverage may be used for public safety.The D2D communication outside a network coverage may be used for onlythe public safety. D2D communication in a BS coverage may be executedbased on a BS.

For example, a BS 200 may transmit D2D resource allocation informationto the first UE 210 located in the BS coverage. The D2D resourceallocation information may include allocation information associatedwith a D2D communication resource for D2D communication between thefirst UE 210 and another UE (for example, a second UE 220).

The first UE 210 that receives the D2D resource allocation informationfrom the BS, may transmit the D2D resource allocation information to thesecond UE 220 outside the BS coverage. The second UE 220 may be a UElocated outside the BS coverage, from the perspective of the BS 200 of afirst cell. The first UE 210 and the second UE 220 may execute D2Dcommunication based on the D2D resource allocation information.Particularly, the second UE 220 may obtain information associated withthe D2D communication resource of the first UE 210. The second UE 220may receive traffic data and/or control data transmitted from the firstUE 210, through a resource indicated by the information associated withthe D2D communication resource of the first UE 210.

In the D2D communication, a UE may transmit control data to another UE.A separate channel (for example, a Physical Uplink Control Channel(PUCCH)) for transmitting control data may not be defined in the D2Dcommunication. When the control channel is not defined in the D2Dcommunication, a UE may use various methods for transmitting controldata for D2D communication.

Here, in D2D communication, physical layer control data forsynchronization includes information transmitted via a synchronizationchannel, for example, a physical D2D synchronization channel (PD2DSCH).The physical layer control data for data communication includesScheduling Assignment (SA) information and may be provided via a channelhaving a format similar to or the same as a PUSCH format for D2Dcommunication. In D2D communication, practical traffic datadistinguished from the physical layer control data may be expressed asthe term “D2D data.”

Additionally, a method for transmitting upper layer control data otherthan the physical layer control data in D2D communication may bedefined.

In a D2D communication, a UE may operate in a first transmission modeand a second transmission mode. The first transmission mode is a mode inwhich the UE is capable of carrying out D2D communication only when theUE has been assigned resources for a D2D communication from a basestation, where a base station sends a D2D grant to a transmitting UE,which transmits a D2D signal to another UE. The D2D grant provides thetransmitting UE with parameter information that needs to be decided by abase station among pieces of Scheduling Assignment (SA) information thatneeds to be obtained at a receiving UE for D2D data reception in a D2Dcommunication, resource allocation information for the SA, and resourceallocation information for data indicated by the SA. The parameterinformation that needs to be decided by the base station includesresource allocation information for data indicated by the SA. The D2Dgrant is forwarded to a transmitting UE in Downlink Control Information(DCI), and may be carried in Physical Downlink Control Channel (PDCCH)or Enhanced PDCCH (EPDCCH). The D2D grant may be control informationwith its distinct D2D purpose indicated by uplink grant or D2D-RNTIassigned to each UE. The D2D grant may be referred to as SA/data grant.

Meanwhile, the second transmission mode is a mode which enables the UEto perform D2D communication regardless of an instruction from the BS,in which the UE may autonomously select a resource for use amongavailable radio resources for D2D communication to transmit D2D data.When information indicating that a particular cell in the BS is capableof supporting D2D through a System Information Block (SIB)/dedicatedsignaling and D2D resource pool information for the second transmissionmode provided from the BS are present, the UE is allowed to operate inthe second transmission mode only for the particular cell. However, theBS does not allow an operation in the second transmission mode, that is,when the information indicating that the particular cell in the BS iscapable of supporting D2D is present but the D2D resource poolinformation for the second transmission mode is not provided from theBS, the UE is not allowed to operate in the second transmission mode.Further, when the D2D resource pool information for the second mode isvalid only in the RRC connected mode, a UE in the RRC idle mode is notallowed to operate in the second transmission mode even in the presenceof the D2D resource pool information for the second transmission mode.Here, when a UE is positioned out of a network service area, that is,when the UE is in an ‘Any Cell Selection’ mode in which the UE is in theRRC idle mode but selects no service-enabled cell, the UE may operate inthe second transmission mode using the D2D resource pool information forthe second transmission mode stored in an Universal Subscriber IdentityModule (USIM) Integrated Circuit Card (UICC) of the UE or the D2Dresource pool information for the second transmission mode received fromthe BS in a service area of a previous network.

In the wireless communication system, the UE reports a status of abuffer thereof to the BS in order to allocate a resource needed totransmit uplink (UL) data (data to be transmitted to the BS) present inthe buffer of the UE, and the BS schedules a resource to be allocate toeach UE based on the information on the status of the buffer reported bythe UE.

Thus, in supporting D2D communication according to an exemplaryembodiment, the BS may need to schedule resources necessary forin-coverage UEs to transmit data via D2D communication. To this end, theBS may need to know how much data to be transmitted via D2Dcommunication (hereinafter, referred to as “D2D data”) is in a UEbuffer. Accordingly, there is suggested a method of a UE notifying theBS of how much data is in a UE buffer to be transmitted via D2Dcommunication.

To this end, the wireless communication system according to an exemplaryembodiment supports a form that a UE reports to the BS a buffer statusregarding UL data (data to be transmitted to the BS) through a BSR andprovides a form and criteria for reporting a buffer status regarding D2Ddata for efficient scheduling of resources needed to transmit D2D data.

FIG. 3 illustrates a process that a UE transmits a ProSe-BSR to the BSin order to transmit D2D data in the first transmission mode in thewireless communication system according to an exemplary embodiment.

When a UE capable of performing D2D communication in the wirelesscommunication system has data to transmit via a D2D link in a DRBconfigured for D2D, a BSR on the D2D data is triggered in operationS310. According to an exemplary embodiment, a BSR on D2D data isreferred to as a ProSe-BSR hereinafter. The ProSe-BSR refers to a BSRfor D2D communication, which is distinguished from a BSR defined andused for the current wireless communication system.

When the ProSe-BSR is triggered, the UE transmits a scheduling request(SR) to the BS in order to induce allocation of a resource fortransmission of the D2D data and ProSe-BSR in operation S320 andreceives an UL grant in response to the SR from the BS in operationS330. Here, the SR is transmitted to the BS via a PUCCH. As the SR, anSR used in a conventional wireless communication system may be used, ora resource additionally allocated by the BS as an SR for D2D purposes,which is distinguished from the SR, may be used. When the SR for D2Dpurposes is distinguished by definition from the conventional SR, the SRmay be distinguished as an ProSe-BSR. For convenience of description, anSR and a ProSe-BSR are collectively referred to as an SR.

When the SR is triggered, the SR is pending until the SR is cancelled.On the contrary, when the UL grant does not accommodate all pending datafor transmission or a Media Access Control Protocol Data Unit (MAC PUD)is configured and includes a ProSe-BSR formed in such a buffer statusthat up to a last occurring event is included, the UE cancels allpending SRs and stops a timer (sr-ProhibitTimer) not to transmit theSRs.

In detail, when the SR is triggered and there is no currently pendingSR, the UE sets an SR counter (SR COUNTER) value to 0. However, in acase where the SR is pending, there is a valid PUCCH resource forsending the SR in a current Transmission Time Interval (TTI), thecurrent TTI is not part of a measurement gap, and the sr-ProhibitTimeris not running, when the SR COUNTER value is smaller than the maximumnumber of SR transmission times, the UE increases the SR counter valueby 1, directs the physical layer to transmit an SR signal through thePUCCH, and starts the sr-ProhibitTimer. However, when the SR COUNTERvalue is equal to or greater than the maximum number of transmissiontimes, the UE notifies the RRC of release of the PUCCH and SRS andclears all configured DL allocations and UL grants. Then, the UEinitializes a random access procedure and cancels all pending SRs.

Meanwhile, when the SR is pending but there is no available UL-SCHresource for transmission in any TTI, the UE initializes the randomaccess procedure and cancels all pending SRs. Thus, the ProSe-BSR may betransmitted to the BS through the random access procedure.

When the UL grant in response to the SR is received, the UE transmitsthe ProSe-BSR to the BS in operation S340. When a D2D grant in responseto the ProSe-BSR is received from the BS in operation S350, the UEtransmits data to a destination UE using a resource allocated fortransmission of the D2D data in operation S360. As described above, theProSe-BSR is for the UE to notify the serving BS of information on theamount of data to be transmitted present in the D2D link buffer.According to an exemplary embodiment, for example, the ProSe-BSRprocedure is performed after the SR is transmitted. However, when the UEreceives a sufficient UL grant to transmit the ProSe-BSR beforetransmission of the SR, the ProSe-BSR may be transmitted beforetransmission of the SR.

The BS configures a periodic BSR timer (periodicBSR-Timer) and aretransmission BSR timer (retxBSR-Timer) for the ProSe-BSR through asignaling defined in the RRC layer to control the ProSe-BSR procedurewith respect to a logical channel in each UE. For each UE, a LogicalChannel Group (LCG) may be optionally configured by an RRC signal by aneNB, and the ProSe-BSR may be performed for an LCG including a logicalchannel for D2D communication (hereinafter, “LCG”). The LCG is setseparately from an LCG that is a destination of a BSR for the wirelesscommunication system. For instance, an LCG for a ProSe-BSR and an LCGfor a conventional BSR are set separately.

Here, the LCG as the destination of the BSR for the wirelesscommunication system are formed of only logical channels (DCCH and DTCH)set for data transmission in the wireless communication system, and thelogical channels (LCs) may have indexes of 0 to 11. On the contrary, theLCG as the destination of the BSR for D2D communication are formed ofonly LCs (PTCHs) set for D2D data transmission, and the LCs for D2Dcommunication may have indexes of 0 to 11 independently of the indexesof the LCs for the wireless communication system (0 to 11).

Additionally, the BS may set the periodic timer/retransmission timer forthe ProSe-BSR through an RRC with respect to each UE separately from aBSR for the wireless communication system.

A UE configures a ProSe-BSR based on pieces of buffered data in each LCGin each UE. Up to four LCGs may be configured in a UE. As a ProSe-BSRformat, there may be a short BSR for reporting a buffer statuscorresponding to one LCG, a long BSR for reporting a buffer statuscorresponding to four LCGs, or a truncated BSR. A BSR format will bedescribed later.

For the ProSe-BSR procedure, a UE may consider a suspended Radio Bearer(RB) as a ProSe-BSR destination and necessarily considers allunsuspended RBs as ProSe-BSR destinations. ProSe-BSRs may be dividedinto a regular ProSe-BSR, a padding ProSe-BSR, and a periodic ProSe-BSR.

The regular ProSe-BSR is triggered when data transmittable to a logicalchannel included in an LLG is present in an RLC entity or PDCP entityand there is UL data transmittable to a logical channel having a higherpriority than other logical channels already including transmittabledata. Further, the regular ProSe-BSR is also triggered when aretxBSR-Timer for a ProSe-BSR expires and a UE includes transmittabledata in a logical channel of an LCG.

The padding ProSe-BSR is triggered when the number or padding bitsremaining after allocation of a UL resource and a resource for paddingBSR transmission for the wireless communication system is equal to orgreater than a size for ProSe-BSR transmission.

Alternatively, the padding ProSe-BSR is triggered when the number orpadding bits remaining after allocation of a UL resource and a resourcefor BSR transmission for the wireless communication system is equal toor greater than the size for ProSe-BSR transmission.

The periodic ProSe-BSR is triggered when a periodicBSR-Timer for theProSe-BSR expires.

The regular ProSe-BSR and the periodic ProSe-BSR are transmitted in along ProSe-BSR format when more than one LCG (at least two or more LCGs)have data to transmit in a TTI in which the ProSe-BSRs are transmitted.Otherwise (when only one LCG has data to transmit), the regularProSe-BSR and the periodic ProSe-BSR may be configured and transmittedin a short BSR format. The padding ProSe-BSR is configured andtransmitted in a truncated ProSe-BSR format with respect to an LCGincluding a logical channel having a top priority in data transmissionwhen the number of padding bits included in an MAC PDU is equal to orgreater than the total size of a short ProSe-BSR and a sub-header of theshort ProSe-BSR but is smaller than the total size of a long ProSe-BSRand a sub-header of the long ProSe-BSR and more than one LCG has data totransmit in an TTI in which the padding ProSe-BSR is transmitted. Inother cases, the padding ProSe-BSR is transmitted in a short ProSe-BSRformat. Alternatively, when only a short ProSe-BSR format is possible asa ProSe-BSR format, the padding ProSe-BSR may always be transmitted inthe short ProSe-BSR format.

Meanwhile, the padding ProSe-BSR is transmitted in a long ProSe-BSRformat when the number of padding bits is equal to or greater than thetotal size of a long ProSe-BSR and a sub-header of the long ProSe-BSR.Alternatively, only a short ProSe-BSR format is possible as a ProSe-BSRformat, the padding ProSe-BSR is always transmitted in the shortProSe-BSR.

Meanwhile, a UE performs the ProSe-BSR procedure when at least oneProSe-BSR is triggered and is not cancelled. When an UL resource for newtransmission is allocated in a current TTI, the UE instructsmultiplexing and assembly procedures for generation of a ProSe-BSR MACcontrol element, starts or restarts the periodicBSR-Timer for theProSe-BSR, and starts or restarts the retxBSR-Timer for the ProSe-BSR.Here, a procedure of starting or restarting the periodicBSR-Timer forthe ProSe-BSR is excluded when a truncated ProSe-BSR is generated. Whenthe UL resource for new transmission is not allocated in the currentTTI, a regular ProSe-BSR is triggered.

Here, one MAC PDU includes only one ProSe-BSR MAC control element evenwhen a plurality of ProSe-BSRs is triggered. Also, when it is possibleto transmit a regular ProSe-BSR or periodic ProSe-BSR, the regularProSe-BSR or periodic ProSe-BSR is always a priority over the paddingProSe-BSR. Also, when reception of an indication which instructstransmission of new data with respect to all UL-SCHs is verified, the UErestarts the retxBSR-Timer. All triggered ProSe-BSRs need to becancelled when the ProSe-BSRs are included in the MAC PDU.

The UE transmits one regular or periodic ProSe-BSR in one TTI. Also,according to an exemplary embodiment, the ProSe-BSR may be transmittedin the same TTI as a BSR of the wireless communication system. Forexample, in the same sub-frame (TTI), a ProSe-BSR for a D2D service anda conventional BSR for a general data service may be simultaneouslytransmitted. Here, information on the BRSs may be distinguished throughLCIDs.

When the UE receives a request for transmission of a plurality of MACPDUs in one TTI, one padding ProSe-BSR may be included in random MACPDUs not including a regular or periodic ProSe-BSR. Thus, one paddingProSe-BSR may be included in random MAC PDUs including a regular orperiodic ProSe-BSR for the wireless communication system. That is,although a padding BSR has a priority over a padding ProSe-BSR, thepadding ProSe-BSR may be included first merely in a random MAC PDUincluding a regular or periodic BSR. The padding ProSe-BSR may also beincluded when a request for transmission of a single MAC PDU in one TTIis received. All ProSe-BSRs always reflect a buffer status after the UEconfigures MAC PDUs to transmit based on a previously received D2D granton the basis of a TTI in which the ProSe-BSR is transmitted. Each LCGreports one buffer status value in each TTI, and the buffer status valueis reported through a ProSe-BSR with respect to the LCG in allProSe-BSRs. That is, in the same TTI, one ProSe-BSR value is transmittedby each LCG, and the buffer status with respect to the LCG value is thesame in all ProSe-BSRs transmitted in the same TTI. Meanwhile, thepadding ProSe-BSR is not allowed to cancel the regular or periodicProSe-BSR. The padding ProSe-BSR is triggered with respect to aparticular MAC PDU, and is cancelled when the particular MAC PDU isgenerated.

However, in a wireless communication system, all triggered BSRs arecanceled when an uplink (UL) grant is received which indicates that itis possible to send all pending data but impossible to additionally senda BSR MAC control element. Further, all triggered BSRs are canceled whenthe BSRs are included in an MAC PDU. That is, in the wirelesscommunication, a triggered BSR may be canceled only in the followingcircumstances.

1. When a sub-frame is capable of accommodating all data to betransmitted via an uplink using a resource secured through an UL grantbut is incapable of including a BSR MAC CE including an MAC sub-header.

In this case, a UE cancels a triggered BSR and transmits all data to betransmitted through the uplink via the uplink.

2. When a BSR is included in an MAC PDU for uplink transmission.

As described above, in D2D communication, a base station (BS) may needto schedule resources needed for in-coverage UEs to transmit data viaD2D communication. To this end, a UE may notify the BS through a BSR ofhow much data to be transmitted via D2D communication (hereinafter,referred to as “D2D data”) is in a UE buffer. However, in an exceptionalcase (for example, when a UE is incapable of maintaining an RRCconnection with a BS), the UE may need to transmit D2D data through aresource that the UE autonomously selects, instead of being allocated bythe BS a resource necessary to transmit data via D2D communication. Tothis end, a first transmission mode (Transmission Mode 1) and a secondtransmission mode (Transmission Mode 2) may be supported in D2Dcommunication.

However, when the UE is set to or operates in the second transmissionmode, a resource for D2D transmission may not be controlled by a BS.Thus, the BS may need to reserve the resource for D2D transmission. Thereserved resource may be determined based on the number of UEs capableof operating in the second transmission mode and the estimated resourceconsumption of each UE.

If a greater amount of resources are reserved for the secondtransmission mode, due to a limit to the overall amount of resources,the amount of resources available for normal wireless communication (forexample, LTE communication) decreases, which may affect the overalltransfer rate of the system. Thus, the BS may reduce the number of UEscapable of operating in the second transmission mode in order todecrease the amount of resources reserved for the second transmissionmode. To support a decrease in the number of UEs capable of operating inthe second transmission mode, the BS may allow a UE to operate in thesecond transmission mode only in an exceptional case.

For this, in a case where a D2D transmission mode set for a UE ischanged from the first transmission mode to the second transmission modeor a UE set to the first transmission mode operates in the secondtransmission mode, when there is an already triggered BSR, it needs tobe determined how to operate the triggered BSR.

FIGS. 4 to 8 are flowcharts illustrating a method for operating a BSRfor D2D communication by a UE according to an exemplary embodiment.Hereinafter, in a case where a D2D transmission mode set for a UE ischanged from the first transmission mode to the second transmission modeor a UE set to the first transmission mode operates in the secondtransmission mode, a method for operating a triggered BSR will bedescribed with reference to FIGS. 4 to 8.

A UE supporting D2D communication may perform D2D communication when aUE user sets up the UE to be ready for D2D communication through a userinterface (UI). Alternatively, a network (for example, a D2D servermanaging a ProSe Identifier (ID) and a ProSe Application ID of a UEusing D2D communication, a serving BS of the UE, or the like) mayultimately determine D2D communication of the UE set up by the UE userto be ready for D2D communication. That is, although the UE is set up bythe UE user to be ready for D2D communication, the UE may perform D2Dcommunication only when the network allows the UE to perform D2Dcommunication. Information on availability of D2D communication may bedisplayed on a screen of the UE.

A resource for D2D communication may be allocated by a UE serving toallocate a resource for D2D communication in D2D communication(hereinafter, cluster head) or by a BS. In this case, a UE needs toreport a BSR on D2D data to the BS or the cluster head when D2Dcommunication is performed. For convenience of description, the BS andthe cluster header are collectively referred to as a BS hereinafter.Also, a BSR on D2D data is referred to as a ProSe-BSR.

In one embodiment with reference to FIG. 4, a ProSe-BSR can be triggeredby the same triggering events as that of a BSR for uplink data inoperation S410. Herein, the BSR shall be triggered if any of thefollowing events occur:

-   -   UL data, for a logical channel which belongs to a LCG, becomes        available for transmission in the RLC entity or in the PDCP        entity and either the data belongs to a logical channel with        higher priority than the priorities of the logical channels        which belong to any LCG and for which data is already available        for transmission, or there is no data available for transmission        for any of the logical channels which belong to a LCG, in which        case the BSR is referred below to as “Regular BSR”;    -   UL resources are allocated and number of padding bits is equal        to or larger than the size of the Buffer Status Report MAC        control element plus its subheader, in which case the BSR is        referred below to as “Padding BSR”;    -   retxBSR-Timer expires and the MAC entity has data available for        transmission for any of the logical channels which belong to a        LCG, in which case the BSR is referred below to as “Regular        BSR”;    -   periodicBSR-Timer expires, in which case the BSR is referred        below to as “Periodic BSR”.

For this, a different LCID of the ProSe-BSR may be used from that of theBSR for the conventional uplink data. For instance, the ProSe-BSR may beallocated a new LCID to distinguish a short ProSe-BSR, a truncatedProSe-BSR, and a long ProSe-BSR. Alternatively, one ProSe-BSR format maybe used as a BSR on D2D data and a new LCID representing the format maybe used.

When the ProSe-BSR is triggered, a UE may determine whether a currentD2D transmission mode of the UE is the second transmission mode inoperation S420. When the D2D transmission mode is the secondtransmission, the UE may cancel the triggered ProSe-BSR in operationS430. When the D2D transmission mode is not the second transmissionmode, the UE may transmit a ProSe-BSR MAC CE to the BS as included in anMAC PDU when an uplink resource is allocated based on the triggeredProSe-BSR in operation S440.

For example, when the UE changes the D2D transmission mode thereof fromthe first transmission mode to the second transmission mode due to theexceptional case according to an exemplary embodiment (a timer relatingto an RRC re-establishment procedure and/or a timer relating to handoveris running or started) or a case where the BS allows the UE to changethe D2D transmission mode to the second transmission mode, the UE maycancel all ProSe-BSRs triggered for an operation in the firsttransmission mode. Here, the first transmission mode may refer to a modein which a UE performs D2D communication using a resource allocationscheduled by the BS, and the second transmission mode may refer to amode in which a UE performs D2D communication using second transmissionmode resource pool information for D2D communication. That is, alltriggered ProSe-BSRs may be cancelled when the D2D communication mode ofthe UE is changed from the first transmission mode to the secondtransmission mode due to the exceptional case and when the UE or BSchanges the D2D communication mode from the first transmission mode tothe second transmission.

A UE operating in the first transmission mode and a UE operating in thesecond transmission mode need to verify D2D resource pool informationbelow included in reception resource pool information commRxPool andnormal common transmission resource pool informationcommTxPoolNormalCommon in order to receive D2D data. Here, the receptionresource pool information is information indicating resources allowedfor reception by D2D UEs operating in RRC idle and connected modes, andthe normal common transmission pool information is informationindicating resources allowed for transmission by a D2D UE operating inthe RRC idle mode.

-   -   SA resource pool information    -   Data resource pool information (included in the normal common        transmission resource pool but not included in a resource pool        for the first transmission mode among reception resource pool        configuration information)    -   TDD configuration information (included in the reception        resource pool information only in the case of a neighbor cell        and a TDD operation)

In addition, transmission resource pool informationcommTxPoolExceptional for an exceptional case, defined separately fromthe foregoing resource pool information, may be defined for theexceptional case.

The all resource pool information may be transmitted to all UEs in acorresponding cell through a broadcast channel, and a UE intended toperform a D2D communication operation needs to perform a monitoringoperation for receiving D2D data that the UE desires to receive throughradio resources based on all pieces of the resource pool information.The reception resource pool information may include transmissionresource pool information dedicated by the BS to a UE operating in theRRC connected mode in the cell for a transmission operation in the firsttransmission mode or second transmission mode. The dedicatedtransmission resource pool information for the first transmission modeor second transmission mode needs to be set up such that physicalresources in respective cells do not collide with each other in order toavoid mutual interference. Also, when resources allocated for ProSecommunication are insufficient so that collisions between resourcesoccur, BSs need to cooperate with each other so that characteristics,such as cyclic prefix length, of transmission signals set up for therespective cells are the same as each other to limit a mutualinterference rage to the same resource range.

Here, the timer relating to the RRC re-establishment procedure may be aT311 timer, the timer relating to the handover may be a T304 timer, andthe respective timers may be defined as in Tables 1 and 2.

TABLE 1 Timer Start Stop At expiry T311 Upon initiating Selection of aEnter RRC_IDLE the RRC connection suitable E-UTRA re-establishment cellor a cell procedure using another RAT.

TABLE 2 Timer Start Stop At expiry T304 Reception of Criterion forsuccessful completion In case of cell change order from E-RRCConnectionReconfiguration of handover within E-UTRA, UTRA or intraE-UTRA handover, message including the handover to E-UTRA or cell changeinitiate the RRC connection re- MobilityControl Info or order is met(the criterion is establishment procedure; In case of reception ofspecified in the target RAT in case handover to E-UTRA, perform theMobilityFromEUTRACommand of inter-RAT) actions defined in thespecifications message including applicable for the source RAT.CellChangeOrder

Referring to Table 1, the T311 timer is started when an RRC connectionre-establishment procedure is initiated. The T311 timer is stopped whena suitable Evolved UMTS Terrestrial Radio Access (E-UTRA) cell isselected or a cell using another Radio Access Technology (RAT) isselected. The T311 timer expires when going to an RRC idle state.

Herein, the UE shall initiate the RRC connection re-establishmentprocedure, upon detecting radio link failure (RLF). According to theinitiation of the RRC connection re-establishment procedure, the UEshall start timer T311. Also the UE shall stop timer T311 when the UEselects a suitable E-UTRA cell for the RRC connection re-establishment.

Referring to Table 2, the T304 timer is started when an RRC connectionreconfiguration message including mobility control information isreceived or a message including cell change order,MobilityFromEUTRACommand message, is received. Here, the RRC connectionreconfiguration message including mobility control information may alsobe referred to as a handover command message. That is, the T304 timer isstarted during handover in an LTE network or inter-RAT handover to anLTE system and is stopped when the handover is successfully completed.

Meanwhile, the UE may reset a periodic BSR timer (periodic ProSeBSR-timer) and a retransmission BSR timer (retx ProSe BSR-timer)configured for a ProSe-BSR when the ProSe-BSR is cancelled. The resetperiodic BSR timer and retransmission BSR timer configured for theProSe-BSR may be set to a start state when the D2D transmission mode ofthe UE is changed from the second transmission mode to the firsttransmission mode. Herein, the ‘reset’ for the periodic BSR timer andthe retransmission BSR timer of the ProSe-BSR refers to stop and go tozero of a timer value on the periodic BSR timer and the retransmissionBSR timer of the ProSe-BSR. That is, it is same operation that theperiodic BSR timer and the retransmission BSR timer of the ProSe-BSR.shall be stopped and start or restart from 0 timer point on the periodicBSR timer and the retransmission BSR timer of the ProSe-BSR.

Further, as illustrated in FIG. 5, when the UE changes the D2Dtransmission mode of the UE from the second transmission mode to thefirst transmission mode in operation S510, the UE may determine whetherresource pool information for the second transmission mode is present inoperation S520. Here, when the D2D transmission of the UE is changed dueto the exceptional case, the D2D transmission mode may be changed fromthe first transmission mode to a fallback mode in operation S520.Although the fallback mode is defined as the same D2D UE operation asthe second transmission mode, resource pool information for the fallbackmode may be transmission resource pool information commTxPoolExceptionalfor an exceptional case, which is defined for the exceptional caseseparately from the resource pool information. Thus, when the D2Dtransmission mode of the UE is changed from the first transmission modeto the fallback mode, it may be further determined whether the resourcepool information for the fallback mode is present in operation S520.Here, resource pool information preferentially applied in the fallbackmode may be the normal common transmission resource pool information orthe transmission resource pool information for the exceptional case withtransmission resource pool information of commTxPoolExceptional. When atleast one of the two pieces of resource pool information is present, theUE may exclude all logical channels (ProSe Communication TrafficChannels (PTCHs)) configured for D2D communication between UEs from anobject of a triggered ProSe-BSR so that a ProSe-BSR is no longertriggered in operation S530. However, when neither of the two pieces ofresource pool information are present, the UE is incapable of operatingin the second transmission mode and thus may suspend data transmissionvia the PTCHs in operation S540.

Subsequently, as shown in FIG. 6, when the exceptional case is released(for example, the timer relating to the RRC re-establishment procedureand/or the timer relating to the handover is stopped) so that the D2Dtransmission of the UE is changed from the second transmission mode tothe first transmission mode in operation S610, the UE may start thereset periodic BSR timer and retransmission BSR timer and transmit a BSRon data to be transmitted via D2D communication. That is, the UE mayrestart a normal ProSe-BSR procedure in operation S620.

In a case of handover, when the UE prepares handover so as to change toand operate in the first transmission mode immediately after handover toa target BS is completed (immediately after the T304 timer is stopped),that is, when a source BS transmits a handover request message includingUE information including ProSe configuration information, such as thecurrent transmission mode of the UE, to the target BS and the target BStransmits a handover permission message to the source BS in response tothe handover request message, ProSe configuration information necessaryfor an operation in the first transmission mode is included attransmission. The configuration information may include D2D-RNTIinformation, ProSe-BSR related configuration information, and SA poolinformation.

In another embodiment, as illustrated in FIG. 7, when the D2Dtransmission mode of the UE is changed from the first transmission modeto the second transmission mode due to the exceptional case in operationS710, the UE may determine whether to cancel a ProSe-BSR depending onwhether there is data to be transmitted via D2D communication. Forexample, when the timer relating to the RRC re-establishment procedureand/or the timer relating to the handover is running and thus the D2Dtransmission mode of the UE is changed from the first transmission modeto the second transmission mode, the UE may cancel the ProSe-BSR onlywhen there is no data to be transmitted via D2D communication inoperation S720. Here, all the ProSe-BSR timers (periodic ProSe-BSR timerand retransmission ProSe-BSR timer) may be reset and not be starteduntil the UE changes the D2D transmission mode to the first transmissionmode. Herein, the ‘reset’ for the periodic BSR timer and theretransmission BSR timer of the ProSe-BSR refers to stop and go to zeroof a timer value on the periodic ProSe-BSR timer and the retransmissionProSe-BSR timer of the ProSe-BSR. In this case, the UE may alsodetermine whether the resource pool information for the secondtransmission mode is present. When the resource pool information for thesecond transmission mode is present, the UE may exclude all PTCHs froman object of a triggered ProSe-BSR. When the resource pool informationfor the second transmission mode is absent, the UE may suspend datatransmission via all PTCHs.

In still another embodiment, as illustrated in FIG. 8, the UE, which isin the first transmission mode, may operate in the second transmissionmode without changing the D2D transmission mode in operation S810. Thiscase may occur when the BS does not propose the D2D transmission mode ofthe UE and/or at least one of the timer relating to the RRCre-establishment procedure and the timer relating to the handover isrunning or started. In this case, when a D2D grant is received from theBS, the UE transmits D2D data in the first transmission mode even thoughthe UE is capable of transmitting D2D data in the second transmissionmode. That is, the UE may maintain the first transmission mode as theD2D transmission mode. In this case, although the UE starts to operatein the second transmission mode, the UE is in the first transmissionmode and thus may not cancel triggering of a ProSe-BSR. However, the UEdetermines the presence of data to be transmitted via D2D communication,and may cancel the triggered BSR when there is no data to be transmittedvia D2D communication in operation S820.

Meanwhile, when the exception case is released but the RRC connectedmode is not maintained (that is, when an RLF related timer and/or thetimer relating to the handover expires), the UE may change the D2Dtransmission mode to the second transmission mode. The UE may transmitD2D data based on obtained D2D resource pool information for the secondtransmission mode.

Alternatively, in the exceptional case, the UE may cancel a ProSe-BSRthrough an operation of resetting an MAC entity. The MAC entity may bereset by a request of an upper layer. The operation of resetting the MACentity includes the following operations.

-   -   Suspend all running timers.    -   Set values of new data indicators (NDIs) with respect to all UL        HARQ processes other than UL HARQ processes related to a PSCH to        0.    -   Cancel a triggered SR and/or ProSe-SR procedure, if any.    -   Cancel a triggered BSR and/or ProSe-BSR procedure, if any.    -   Flush soft buffers with respect to all DL HARQ processes other        DL HARQ processes related to the PSCH.    -   Consider transmission of a random TB received subsequently with        respect to each DL HARQ process other than the DL HARQ processes        related to the PSCH as very first transmission.

Here, a ProSe communication shared channel (PSCH) is a transport channelfor D2D data transmitted from an MAC layer to a physical layer.

If the exceptional case occurs due to the start of the RRCre-establishment procedure, the UE suspends all DRBs other than DRBsrelated to a PTCH as a logical channel related to transmission of D2Ddata when the RRC re-establishment procedure is started. If theexceptional case occurs due to the start of the handover procedure (theRRC reconfiguration message including mobility control information(MCI), the UE re-establishes RLCs and PDCPs in all DRBs other than theDRBs related to the PTCH as the logical channel related to transmissionof D2D data when the handover procedure is started.

When an MAC entity for D2D communication is separated from an MAC entityfor radio communication, the MAC entity for D2D communication is resetand a ProSe-BSR is cancelled in the exceptional case.

Here, the UE, in which the exceptional case is in progress, does nottrigger a ProSe-BSR even when transmittable data is present in an RLCand PDCP of a DRB related to the PTCH. That is, triggering a ProSe-BSRis possible only not in the exceptional case and when the UE is set tooperate in the first transmission mode and is in the RRC connected mode.

FIG. 9 is a flowchart illustrating a method for operating a BSR for D2Dcommunication by a BS according to an exemplary embodiment.

Referring to FIG. 9, a BS may transmit a second transmission modepermission message to a first UE in a coverage in operation S910. Here,the first UE may be a UE a D2D transmission mode of which is set to thefirst transmission mode. The first UE receiving the second transmissionmode permission message may change the D2D transmission mode to thesecond transmission mode in the afore-described exceptional case.

When the first UE is changed to the second transmission mode, the BS mayreceive a ProSe-BSR from a second UE set to the first transmission modeother than the first UE in operation S920. Then, the BS may allocate aresource for D2D communication to the second UE based on the receivedProSe-BSR in operation S930, and the second UE may perform D2Dcommunication with a target UE using the allocated resource.

Alternatively, when an RRC reconfiguration message including mobilitycontrol information (MCI) is received from the BS, a UE may release aconfiguration for a transmission resource among configurations for ProSecommunication operations, which is for indicating that configurationinformation for the first transmission mode or second transmission modeconfigured by a source BS is no longer valid since the UE moves to atarget BS through handover.

For the release operation, release information may be transmitted asincluded in configuration information on the transmission resource inthe RRC reconfiguration message. Alternatively, when the RRCreconfiguration message including the mobility control information (MCI)is received from the BS, the UE may perform an operation of autonomouslyreleasing the configuration information on the transmission resource inthe RRC reconfiguration message.

As described above, when the configuration for the transmission resourcefor the first transmission mode or the second transmission mode isreleased, the UE operates in the second transmission mode in datatransmission using normal common transmission resource pool informationreceived through system information. Quality of service (QoS) of ProSecommunication may be maintained even in the RRC idle mode using anavailable ProSe communication transmission resource, instead of theconfiguration information for the first transmission mode or the secondtransmission mode configured by the source BS which is released due to ahandover operation as described above.

Alternatively, when the configuration for the transmission resource forthe first transmission mode or the second transmission mode is released,the UE may operate in the fallback mode in data transmission usingtransmission resource pool information for an exceptional case receivedthrough system information. Although the configuration information forthe first transmission mode or the second transmission mode configuredby the source BS is released due to a handover operation, the UEmaintains the RRC connected mode, QoS of ProSe communication may bemaintained using the transmission resource pool information for theexceptional case available even in the RRC connected mode.

Here, the used transmission resource pool information may be resourcepool information for ProSe communication provided by the target BS tothe source BS (BS transmitting the RRC reconfiguration message includingMCI) and be provided as included in the RRC reconfiguration messageincluding MCI, and the UE may perform a transmission related operationbased on the information provided by the target BS.

When there is no transmission resource pool information provided fromthe target BS or transmission resource pool information is not provided,transmission resource pool information in the system informationpreviously received from the source BS may be used.

The UE may maintain the second transmission mode or fallback mode untilthe UE receives and reflects information for a configuration for atransmission resource among the configurations for the ProSecommunication operations from the target BS after the handover iscompleted.

Alternatively, when the configuration for the transmission resource forthe first transmission mode or the second transmission mode is released,the UE may maintain a reception operation but suspend all transmissionoperations. That is, the UE may operate in a ProSe transmissionsuspending mode. Since the configuration information for the firsttransmission mode or the second transmission mode configured by thesource BS is released due to the handover as described above, a ProSetransmission operation is impossible. This is because when a physicalresource in a transmission resource pool, which is dedicated by thesource BS to a UE operating in the RRC connected mode for a transmissionoperation in the first transmission mode or the second transmissionmode, collides with a physical resource in a transmission resource pooldedicated to the target BS, a ProSe communication signal transmittedbased on transmission resource pool information on the source BS acts asinterference with another ProSe communication signal in the target BS.Thus, the UE suspends the transmission operation in order to remove suchan interference factor.

The UE may the transmission suspending mode until the UE receives andreflects the information for the configuration for the transmissionresource among the configurations for the ProSe communication operationsfrom the target BS after the handover is completed.

Here, the source BS and the target BS may be different from or the sameas each other. When the source BS is the same as the target BS, afrequency band may or may not be changed due to handover. A BS orfrequency band is not changed despite handover when the BS determinesthat RRC parameters of the BS and a UE are inconsistent with each other,the BS determines that DL or UP synchronization is not properlyachieved, or the BS changes a key value shared between the BS and a UEfor security.

FIG. 10 is a block diagram illustrating a wireless communication systemaccording to an exemplary embodiment.

Referring to FIG. 10, the wireless communication system which supportsD2D communication may include a UE 1000 and a BS (or cluster head) 1050.

The UE 1000 may include a radio frequency (RF) unit 1010 and a memory1015. The memory 1015 may be coupled to a processor 1005 and storevarious pieces of information to operate the processor 1005. The RF unit1010 may be coupled to the processor 1005, and may transmit and/orreceive a wireless communication signal. For example, the RF unit 101may receive, from the base station 1050, D2D resource allocationinformation and second transmission mode permission message describedherein. Further, the RF unit 1010 may transmit an uplink signal, such asthe ProSe-BSR described herein, to the base station 1050.

The processor 1005 may implement functions, processes, and/or methodsdescribed herein. Specifically, the processor 1005 may performoperations according to FIG. 4 through FIG. 8.

For example, the processor 1005 may include a ProSe-BSR triggeringdetermination unit 1006, a timer determination unit 1007, and acancellation unit 1008. The ProSe-BSR triggering determination unit 1006determines whether a ProSe-BSR is triggered. The timer determinationunit 1007 determines whether a timer relating to an RRC re-establishmentprocedure and/or a timer relating to handover are running when a currentD2D transmission mode of a UE is the second transmission mode. Upondetecting radio link failure (RLF), the initiation of the RRC connectionre-establishment procedure is initiated, a timer T311 of the RRCconnection re-establishment procedure is started.

The cancellation unit 1008 cancels all triggered ProSe-BSRs when thetimer relating to the RRC re-establishment procedure and/or the timerrelating to the handover are running. For example, when the firsttransmission mode as a Scheduled resource allocation to the secondtransmission mode as a UE autonomous resource selection according to theRRC re-establishment procedure by detecting radio link failure (RLF) ofthe cancellation unit 1008 of the UE, the UE may cancel all ProSe-BSRstriggered for an operation in the first transmission mode.

When a triggered ProSe-BSRs is cancelled, a periodic BSR timer (periodicProSe BSR-timer) and a retransmission BSR timer (retx ProSe BSR-timer)configured for the ProSe-BSRs may be reset. The ‘reset’ for the periodicProSe BSR timer and the retransmission ProSe BSR timer of the ProSe-BSRoperates to stop and go to zero of a timer value on the periodic ProSeBSR timer and the retransmission ProSe BSR timer of the ProSe-BSR.

Meanwhile, the processor 1005 may verify whether resource poolinformation for the second transmission mode or resource poolinformation for the fallback mode is present. Herein, transmissionresource pool information including the commTxPoolExceptional for anexceptional case as the fallback mode is checked by the processor 1005.When at least one of the two pieces of resource pool information,especially the transmission resource pool information including thecommTxPoolExceptional for an exceptional case is present, the processor1005 may exclude all PTCHs configured for D2D communication between UEsfrom an object of a triggered ProSe-BSR so that a ProSe-BSR is no longertriggered. When neither of the two pieces of resource pool informationare present, the processor 1005 is incapable of operating in the secondtransmission mode and thus may suspend data transmission via the PTCHs.

Subsequently, when the timer determination unit 1007 determines that thetimer relating to the RRC re-establishment procedure and/or the timerrelating to the handover is stopped, the processor 1005 determines thatthe exceptional case is released and thus may start the reset periodicBSR timer and retransmission BSR timer and restart a normal ProSe-BSRprocedure.

The memory 1015 may store the resource pool information for the secondtransmission mode, information on whether the second transmission modeis permitted, or the like according to the present specification andprovide the resource pool information for the second transmission modeto the processor 1005 upon a request from the processor 1005.

The base station 1050 may include an RF unit 1055, a processor 1060, anda memory 1065. The memory 1065 may be coupled to the processor 1060, andstore various pieces of information to operate the processor 1060. TheRF unit 1055 may transmit or receive a wireless signal in coupling withthe processor 1060. The processor 1060 may implement functions,processes, and/or method described herein. In exemplary embodimentillustrated above, the operations of a base station 1050 may beimplemented as the processor 1060 and other components. The processor1060 may generate the second transmission mode permission messagedescribed above, and schedule resources for a D2D communication based onthe ProSe-BSR received from a UE.

For instance, the processor 1060 may include a timer value setup unit1061, a ProSe-BSR verification unit 1062, and a D2D resource allocationunit 1063. The timer value setup unit 1061 may set up values of varioustimers specified in the present specification, especially, for the timerT311, the periodic ProSe BSR timer, and the retransmission ProSe BSRtimer of the ProSe-BSR. The timer values set up by the timer value setupunit 1061 may be transmitted to the UE through an RF unit 1055. TheProSe-BSR verification unit 1062 may verify a ProSe-BSR received fromthe UE 1000. The D2D resource allocation unit 1063 may allocate aresource for D2D communication to the UE 1000 operating in a firsttransmission mode based on the ProSe-BSR. The D2D resource allocationunit 1063 also allocates the transmission resource pool information ofcommTxPoolExceptional for the exceptional case as the fallback mode.Herein the all resource pool information including thecommTxPoolExceptional may be transmitted to all UEs in a correspondingcell through a broadcast channel by the processor 1060. According to anexemplary embodiment, the UE 1000 may include a wireless transceiver andone or more processors. The wireless transceiver may be the RF unit 1010or may include the RF unit 1010. The one or more processors may be theprocessor 1005. The processor 1005 may be configured as a singleprocessor or multiple processors.

The wireless transceiver may establish a radio resource control (RRC)connection with an evolved NodeB (eNB) and receive configurationinformation associated with a D2D communication from the eNB. Theconfiguration information may include information of a resourceallocation mode for a D2D data transmission and information of a timerassociated with a BSR for a D2D data transmission. The one or moreprocessors may be configured to determine a radio link failure (RLF), toinitialize a re-establishment process for the RRC connection, to cancelall BSRs for a D2D data transmission in response to the determined RLF,and to initialize the timer associated with a BSR for a D2D datatransmission. The RLF may be associated with a connection problem of theRRC connection, and the canceled BSRs for a D2D data transmission may beassociated with a D2D data transmission scheduled by the eNB.

Further, the one or more processors may initialize a periodic BSR-timerand a retransmission BSR-timer (retx BSR-timer). For example, the one ormore processors may stop the periodic BSR-timer and the retx BSR-timerand set timer values of the periodic BSR-timer and the retx BSR-timer aszero.

The wireless transceiver may receive information of a resource selectionmode. In the resource selection mode, the one or more processors may beconfigured to select a resource for transmitting D2D data from aresource pool indicated by the information of a resource selection mode.

The one or more processors may be configured to determine a change fromthe resource allocation mode for a D2D data transmission to a resourceselection mode when an exceptional case occurs. In the resourceselection mode, the one or more processors may be configured to select aresource for transmitting D2D data from a resource pool. The exceptionalcase may include a case where upon detecting the RLF, the initializationof the re-establishment process for the RRC connection has beeninitiated and a T311 timer associated with the re-establishment processfor the RRC connection has been started.

The wireless transceiver may receive information ofCommTxPoolExceptional for a D2D data transmission for the exceptionalcase. The CommTxPoolExceptional indicates information of a resource poolfor a D2D data transmission when the exceptional case occurs. Thewireless transceiver may receive the CommTxPoolExceptional from the eNBthrough a broadcast channel. The CommTxPoolExceptional is included in aSystem Information Block (SIB). The wireless transceiver may transmitD2D data by selecting, by the processor, a resource based on informationof CommTxPoolExceptional, the CommTxPoolExceptional indicating resourcepool information for a D2D data transmission when the exceptional caseoccurs.

According to an exemplary embodiment, the wireless transceiver mayestablish a radio resource control (RRC) connection with an evolvedNodeB (eNB) and receive configuration information associated with a D2Dcommunication from the eNB, the configuration information comprisinginformation of a resource allocation mode for a D2D data transmission.The one or more processors may operate a UE in a first transmission modefor a D2D communication, the first transmission mode corresponding tothe resource allocation mode. The one or more processors may transitionfrom the first transmission mode to a second transmission mode inresponse to determining the exceptional case. The second transmissionmode corresponds to a resource selection mode in which the UE selects aresource for transmitting D2D data from a resource pool. The one or moreprocessors may cancel at least one triggered BSR for a D2D datatransmission in response to determining the exceptional case, thecanceled BSR for a D2D data transmission being associated with a D2Ddata transmission scheduled by the eNB.

The one or more processors may initialize a timer associated with a BSRfor a D2D data transmission in the second transmission mode. Theinitialized timer restarts when the UE transitions into the firsttransmission mode from the second transmission mode. A periodicBSR-timer and a retransmission BSR-timer (retx BSR-timer) may be resetin response to determining the exceptional case.

The processors may include an application-specific integrated circuit(ASIC), another chipset, a logic circuit, and/or a data processingdevice. The memories may include a Read-Only Memory (ROM), a RandomAccess Memory (RAM), a flash memory, a memory card, a storage mediumand/or another storage device. The RF units may include a basebandcircuit for processing a wireless signal. When an embodiment is embodiedas software, the described scheme may be embodied as a module (process,function, or the like) that executes the described function. The modulemay be stored in a memory, and may be executed by a processor. Thememory may be disposed inside or outside the processor, and may beconnected to the processor through various well-known means.

In the described exemplary system, although methods are described basedon a flowchart as a series of steps or blocks, aspects of the presentinvention are not limited to the sequence of the steps and a step may beexecuted in a different order or may be executed in parallel withanother step. In addition, it is apparent to those skilled in the artthat the steps in the flowchart are not exclusive, and another step maybe included or one or more steps of the flowchart may be omitted withoutaffecting the scope of the present invention.

What is claimed is:
 1. A method comprising: receiving, by a wirelessuser device from a base station, one or more messages comprisinginformation of a resource allocation mode for a direct data transmissionbetween the wireless user device and a different wireless user device;determining, by the wireless user device, a radio link failure (RLF)associated with a problem of a radio resource control (RRC) connection;canceling, based on the determined RLF, buffer status reports (BSRs) forthe direct data transmission, wherein the canceled BSRs for the directdata transmission are associated with a direct data transmissionscheduled by the base station; initializing a timer associated with theBSRs for the direct data transmission; and performing, based on aresource for a direct data transmission in an exceptional case, thedirect data transmission in the exceptional case, wherein the resourcefor the direct data transmission in the exceptional case is based oninformation of CommTxPoolExceptional, and wherein theCommTxPoolExceptional indicates resource pool information for the directdata transmission in the exceptional case.
 2. The method of claim 1,wherein initializing of the timer associated with the BSRs for thedirect data transmission comprises: initializing a periodic BSR-timerand a retransmission BSR-timer (retx BSR-timer).
 3. The method of claim2, wherein initializing of the periodic BSR-timer and the retx BSR-timercomprises: stopping the periodic BSR-timer and the retx BSR-timer; andresetting timer values of the periodic BSR-timer and the retx BSR-timer.4. The method of claim 1, further comprising: receiving information of aresource selection mode; and changing, based on the determined RLF, fromthe resource allocation mode to the resource selection mode.
 5. Themethod of claim 1, further comprising: determining, based on anoccurrence of the exceptional case, a change from the resourceallocation mode to a resource selection mode; and in the resourceselection mode, selecting, based on the resource pool information, theresource for the direct data transmission in the exceptional case,wherein the exceptional case comprises: a case where upon detecting theRLF, an initialization of a re-establishment process for the RRCconnection has been initiated and a T311 timer associated with there-establishment process for the RRC connection has been started.
 6. Themethod of claim 1, further comprising: receiving, via a SystemInformation Block (SIB), the information of the CommTxPoolExceptionalfor the direct data transmission in the exceptional case.
 7. The methodof claim 1, further comprising: receiving, from the base station via abroadcast channel, the information of the CommTxPoolExceptional.
 8. Themethod of claim 1, further comprising initializing a re-establishmentprocess for the RRC connection.
 9. A wireless user device comprising:one or more processors; and memory storing instructions that, whenexecuted by the one or more processors, cause the wireless user deviceto: receive, from a base station, one or more messages comprisinginformation of a resource allocation mode for a direct data transmissionbetween the wireless user device and a different wireless user device;determine a radio link failure (RLF) associated with a problem of aradio resource control (RRC) connection; cancel, based on the determinedRLF, buffer status reports (BSRs) for the direct data transmission,wherein the canceled BSRs for the direct data transmission areassociated with a direct data transmission scheduled by the basestation; initialize a timer associated with the BSRs for the direct datatransmission; and perform, based on a resource for a direct datatransmission in an exceptional case, the direct data transmission in theexceptional case, wherein the resource for the direct data transmissionin the exceptional case is based on information ofCommTxPoolExceptional, and wherein the CommTxPoolExceptional indicatesresource pool information for the direct data transmission in theexceptional case.
 10. The wireless user device of claim 9, wherein theinstructions, when executed by the one or more processors, cause thewireless user device to initialize the timer by initializing a periodicBSR-timer and a retransmission BSR-timer (retx BSR-timer).
 11. Thewireless user device of claim 9, wherein the instructions, when executedby the one or more processors, cause the wireless user device to:receive information of a resource selection mode; and change, based onthe determined RLF, from the resource allocation mode to the resourceselection mode.
 12. The wireless user device of claim 9, wherein theinstructions, when executed by the one or more processors, cause thewireless user device to: determine, based on an occurrence of theexceptional case, a change from the resource allocation mode to aresource selection mode; and in the resource selection mode, select,based on the resource pool information, the resource for the direct datatransmission in the exceptional case, wherein the exceptional casecomprises: a case where upon detecting the RLF, an initialization of are-establishment process for the RRC connection has been initiated and aT311 timer associated with the re-establishment process for the RRCconnection has been started.
 13. A method comprising: receiving, by awireless user device from a base station, one or more messagescomprising information of a resource allocation mode for a direct datatransmission between the wireless user device and a different wirelessuser device; triggering, while the wireless user device is operating inthe resource allocation mode, a buffer status report (BSR) for thedirect data transmission; determining, after the triggering, that atimer associated with a handover of the wireless user device is running;canceling, based on the determining that the timer associated with thehandover of the wireless user device is running, BSRs for the directdata transmission, wherein the canceled BSRs for the direct datatransmission are associated with a direct data transmission scheduled bythe base station and associated with the resource allocation mode; andperforming, based on a resource for a direct data transmission in anexceptional case, the direct data transmission in the exceptional case,wherein the resource for the direct data transmission in the exceptionalcase is based on information of CommTxPoolExceptional, and wherein theCommTxPoolExceptional indicates resource pool information for the directdata transmission in the exceptional case.
 14. The method of claim 13,further comprising: receiving one or more of: a radio resource control(RRC) reconfiguration message comprising mobility control information,or a cell change order message; and starting, based on the RRCreconfiguration message or the cell change order message, the timerassociated with the handover of the wireless user device.
 15. The methodof claim 14, wherein the timer associated with the handover of thewireless user device comprises a T304 timer.
 16. The method of claim 13,further comprising: stopping, after completing the handover, the timerassociated with the handover of the wireless user device; and changing,based on the stopping the timer, from a resource selection mode to theresource allocation mode.
 17. The method of claim 13, furthercomprising: determining, based on an occurrence of the exceptional case,a change from the resource allocation mode to a resource selection mode;and in the resource selection mode, selecting, based on the resourcepool information, the resource for the direct data transmission in theexceptional case, wherein the exceptional case comprises: a case where aT304 timer associated with the handover has been started.
 18. The methodof claim 13, further comprising: receiving, via a System InformationBlock (SIB), the information of the CommTxPoolExceptional for the directdata transmission in the exceptional case.
 19. The method of claim 13,further comprising: receiving, from the base station via a broadcastchannel, the information of the CommTxPoolExceptional.